1. Field of the Invention
The present invention relates to a process for metal surface treatment, and more particularly to an effective process for removing an oxide film, and residues of organic matters, carbon, etc., if any, from a solder surface or a metal surface to be treated, when, for example, a circuit wiring substrate and a semiconductor integrated circuit (LSI) or the like are bonded to each other by soldering, or when plating is applied to a metal surface.
2. Related Art
Heretofore, it has been required that, when a circuit wiring substrate and a semiconductor integrated circuit or the like are bonded to each other by soldering, the surfaces of metal to be bonded be kept clean and there be no solder wettability-inhibiting substances. Furthermore, it has been also required that there be no oxide film, etc. on the surface of metal to be plated and the surface of the metal be kept clean. When an Au wire or ribbon is bonded to the metal surface by ultrasonic welding, the presence of an oxide film on the surface of the metal is a serious problem, and thus the surface of the metal must be kept clean.
Such solder wettability-inhibiting substances include, for example, oxides, chlorides, sulfides, carbonates, and organic compounds. Particularly in the processes for soldering, plating, or bonding of an Au wire or ribbon by ultrasonic welding, most serious inhibiting substance is an oxide film existing on the surface of such metal to be treated, as solder, nickel (Ni), nickel alloys (alloys of nickel with other element or elements).
Generally, the oxide film is chemically converted to a liquid compound by a flux, whereby the metal atoms of the surface of the metal and the metal atoms of a solder have a chance of direct collisions to each other for forming a metal bond by shearing of their outer electron orbits to form an alloy.
In the case of plating, it is impossible to conduct plating if there is an oxide film on the metal surface. For example, in the case of electroplating, which is typical of the plating, the oxide film acts as an insulating film and inhibits the necessary electrical conduction for electroplating, resulting in a failure to conduct plating.
In the case of substitution plating, the oxide film also acts as an inhibitor and there occurs no substitution reaction between the surface of metal to be treated and a plating solution, resulting in a failure to conduct plating.
The oxide film can be removed by treating the surface of the metal with hydrochloric acid or the like before the plating, but hydrochloric acid or the like remains as residues on the treated surface and such residues serve to be a factor of lowering the bonding reliability. Thus, washing of the surface of metal with flons (fluorocarbons) has been so far carried out after the treatment with an acid.
Recently, it has been proposed to use a very small amount of abietic acid (rosin) and adipic acid or the like as a flux without any after-washing of flux residues, but the proposed process is still not satisfactory in the bonding reliability [see "Alumit Technical Journal 19" (1992) and an article "Working mechanism and problems of flux for unnecessitating washing" by N. Kubota of K.K. Nihon Kogyo Gizyutsukaihatsu Kenkusho (Industrial Technology Development Research Institute of Japan, Ltd.)].
On the other hand, a glazing process for irradiating the surfaces of a metallic materials, steel, carbides, etc. with a laser beam, thereby producing materials having a uniform microfine structure or amorphous structure and good corrosion resistance and wear resistance has been proposed and has been applied to processing of metallic materials to be exposed to high temperature and high pressure, such as materials for automobile turbines [see "Laser Processing" (continued part), page 164, by A. Kobayashi, published by Kaihatsu-sha].
It has been also proposed to remove oxide films from metal surfaces by argon sputtering without using any flux or hydrochloric acid.
Furthermore, it has been proposed to form a coating film having a good adhesion without pinholes by roughening a metal surface by blasting, plating the roughened metal surface with an alloy element and then irradiating the plating layer with a laser beam, thereby melting the plating layer (JP-A-63-97382).
Still furthermore, it has been proposed to form a surface layer having a good corrosion resistance and a ready susceptibility to soldering by forming an anodized oxide film on the surface of aluminum or its alloys (JP-A-62-256961).
As a result of extensive study of the above-mentioned prior art, the present inventors have found the following problems:
(1) When a circuit substrate and an integrated circuit or the like as to be bonded to each other by soldering, an oxide film must be removed from their surfaces by a flux before the soldering, and the resulting flux residues must be washed off from their surfaces. In the case of removing oxide films from metal surfaces by washing with an acid before the plating, the resulting acid residues or the metal surfaces cause a later corrosion. Furthermore, an additional drying step is imperative after the washing. PA1 (2) When oxide films are removed from metal surfaces by argon sputtering, the sputtering must be carried out in vacuum. Thus, an additional sputtering apparatus and its complicated operation control are required. There is also such a problem that the argon sputtering has an adverse effect on working elements of electronic parts or electronic devices. PA1 (3) In the glazing process using a laser beam and the laser irradiation process disclosed in the above-mentioned JP-A-63-97382, the surface metal structure is forcedly changed by irradiation with a laser beam of higher energy level and by the resulting melting, thereby endowing a high wear resistance and a high compactness to the metal surface, but an oxide film is inevitably formed on the metal surface in the course of metal surface solidification. PA1 (4) The surface treatment disclosed in the above-mentioned JP-A-62-256961 does not relate to an oxide film-removing technique.